Ignore domain validation for api scans

What does this MR do?

Domain validation is used as a fail safe to ensure that domains that don't want to be scanned will not be scanned by DAST. This doesn't work for API scanning:

• The DAST_WEBSITE url will be a file, or a URL to a specification. This is not the same as the target API service.
• Using default behaviour, the site that hosts the specification will be the one tested for domain validation.
• This MR explicitly turns the check off for API scans. An issue will be created to add the functionality for API scans.

What are the relevant issue numbers?

gitlab-org/gitlab#10928 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer